Willem
Tera Sage

You can create onChange client script on the URL field to validate:

replace your field with the url field

var url = g_form.getValue('your field');

var re = /^(http[s]?:\/\/){0,1}(www\.){0,1}[a-zA-Z0-9\.\-]+\.[a-zA-Z]{2,5}[\.]{0,1}/;
if (!re.test(url)) {
    g_form.showFieldMsg('your field', 'invalid url');
    return false;
}